1. Technical Field
The present invention relates to an actuator, an optical scanner, and an image forming apparatus.
2. Related Art
Optical deflectors intended for application to equipment such as displays and printers using laser beams are required to provide faster scanning. However, improvement in the performances of polygon mirrors or galvano-mirrors used today is limited. Expected now as an alternative optical deflector is a mirror device that is fabricated by processing a silicon substrate using micro electro mechanical systems (MEMS). An MEMS mirror is driven at a high resonance frequency and thus allows formation of high-resolution images.
For example, an optical scanner of equipment such as a printer that performs drawing by optical beam scanning, JP-A-8-322227, as a first related-art example, discloses an actuator that performs two-dimensional light beam scanning.
The actuator disclosed in the first related-art example includes a semiconductor substrate, drive coils, and static magnetic field generating means. The semiconductor substrate has a movable portion and an axial support portion integrally formed thereto. The movable portion includes a frame like outer movable plate and an inner movable plate located within the outer movable plate. The axial support portion has first torsion bars for axially supporting the outer movable plate so as to enable the outer movable plate to swing, and second torsion bars that are orthogonal to the first torsion bars in its axial direction and axially support the inner movable plate so as to enable the inner movable plate to swing. The drive coils are respectively provided on peripheral portions of the outer movable plate and inner movable plate. The static magnetic field generating means applies a static magnetic force to the drive coils. In the actuator, the movable portion is driven by a magnetic field produced by a current flowing through the drive coils, and that one pair of the static magnetic field generating means is provided on one diagonal line of the movable portion so as to face the movable portion.
However, since the drive coils are provided on the movable plate in the actuator disclosed in the first related-art example, the movable plate may be bent by the heat of the coils. Also, since the pair of static magnetic field generating means is provided facing each other with the movable portion therebetween, it is difficult to downsize the apparatus. Moreover, since each of the drive coils is provided on each of the outer and inner movable plates, it is difficult to reduce costs.